A drainmast is used to eject waste water from an aircraft waste system during flight without impingement of the ejected fluid further aft on the body of the aircraft, and to drain the waste system when on the ground.
A drainmast generally comprises a drain tube enclosed within an aerodynamically shaped, drag minimizing fairing that when installed protrudes downwardly from the underside of an aircraft such that its outlet extends into the surrounding airflow stream during flight. It may be swept aftwardly of the aircraft with the direction of airflow. The drain tube is coupled to the aircraft waste water storage tank and is heated within the fairing to avoid freezing. There are two prevalent configurations for drainmasts. One is referred to as a bottom discharge mast and the other configuration is referred to as a rear discharge mast.
In a bottom discharge drainmast, the waste water is ejected generally perpendicularly to surrounding airflow. The mast is generally straight and swept. The axis of the drain tube at the exit forms an acute angle with the axis at the entrance. The angle is usually close to the angle of sweep of the mast.
In a rear discharge drainmast, the mast is swept and wing shaped and tapers into an exit tube at the tip that is roughly parallel to surrounding airflow. Water is ejected parallel to the surrounding airflow from the rear of the mast. Since the axis of the drain tube at the exit is nearly perpendicular to the axis at the entrance, the drain tube is either bent in a smooth curve or is fabricated from two or more straight sections that together form the appropriate angle.
The rear discharge mast is believed to be preferable to the bottom discharge mast. A portion of the water ejected from a bottom discharge mast has a tendency to adhere to the tip of the mast and dribble back driven by surrounding airflow. Much of it can freeze to the mast before it is blown away. In contrast, a properly designed rear discharge mast generates a suction action as surrounding airflow passes around the exit tube. Ejected water tends to be sucked out of the tube into surrounding airflow. Very little if any water adheres to the mast.
An exemplary drainmast is described in U.S. Pat. No. 5,552,576.
The drainmast is exposed to a wide range of operating temperatures depending largely on whether the aircraft is in flight or on the ground. Thus, the drain tube is oftentimes heated to ensure that fluid within the drain tube does not freeze and consequently block the passage of fluid through the drain tube. Heating of the drain tube can require high power densities when operating in flight to prevent fluid from freezing in or on the drain tube. When operated on the ground, the power requirements for heating the drain tube are substantially less than when in flight. As a result, if the drain tube remains heated on the ground at the same power levels as in flight, the drain tube can quickly become overheated. Such overheating can exceed safe limits and can cause damage to the fairing and/or the heater. Additionally, varying flight conditions can also result in overheating conditions.
Accordingly, it has been necessary to provide some form of temperature control for the heated drain tube. In the past, various methods of temperature control have been utilized. For example, the drainmast could include a built in mechanical thermostat. Alternatively, the power provided to the heater could be switched between high power during in flight operation and low power for on the ground operation. According to another approach, a temperature sensor located in the drainmast provided temperature information to the aircraft electronics located outside the drainmast which in turn regulate the power provided to the heater.
The above discussed methods for regulating or controlling the temperature have been subject to one or more shortcomings. For example, mechanical thermostats do not always offer the desired precision and possess thermal lag which can make it difficult to accurately control temperatures. In addition, mechanical thermostats are unable to switch higher currents, have shorter lifetimes and are less reliable then other types of temperature control. Temperature control based on switching between high and low power heating requires complicated switching within the aircraft, multiple level power lines, etc. A temperature sensor located in the drainmast required connection to electronics within the aircraft, thereby increasing the number of electrical and mechanical connections necessary between the drainmast and aircraft, thus complicating assembly.
In view of the foregoing short comings associated with previous drainmasts and the techniques for controlling the drain tube temperature, there is a strong need in the art for an improved drainmast and method for controlling the temperature of the drain tube. In particular, there is a strong need for a drainmast which does not require complicated switching or control form the aircraft. There is a strong need for a drainmast which does not require several electrical connections between the drainmast and the aircraft. In addition, there is a strong need for a drainmast which does not rely on mechanical thermostats.